The present invention relates to apparatuses and methods for controlling a synchronous generator and more particular, to an apparatus and method for controlling a synchronous generator without using a sensor for detecting the position of a rotor in the generator.
In order to drive a synchronous machine, a sensor for detecting the position of a rotor is required in the prior art machine. Meanwhile, there are already proposed several methods of controlling the driving operation of a synchronous machine without using a sensor for detecting the position of a rotor in the machine. Described, for example, in JP-A-9-191698 is a method for estimating a voltage induced in a synchronous machine, estimating a shift angle between the position of a rotor of the machine and an estimated value of the rotor position on the basis of the estimated induced voltage, and correcting the estimated value of the rotor position.
A control system of a converter for driving the synchronous machine generally includes a speed control system and a current control system. The speed control system receives a difference between a detected speed value and a speed command, and generates a current command according to the speed command. The current control system receives a difference between a detected current value and a current command, and generates a voltage command to the converter according to the current command.
However, when a sensor for detecting the position of a rotor is used, a total cost is increased by a cost corresponding to the sensor. Further, since not only the sensor for detection of the rotor position but also wiring signal lines for the sensor are required, this leads to an increase in the entire scale of its machine. Furthermore, when the total length of the signal lines becomes large, the signal lines will tend to easily carry noise and thus a measure to avoid the noise will be required. Even when the sensor for detecting the rotor position is not used, on the other hand, the aforementioned rotor position estimating method in the prior art requires estimation of a shift angle between the rotor position and an estimated value of the rotor position. For this reason, the structure of a rotor position estimating block becomes complicated and thus a calculation time taken for the estimation of the rotor position is also prolonged. In the converter control system in the prior art, a change in a resistive value caused by a change in temperature or the like generates an error. When such an error causes an error in the estimated value of the rotor position, the control accuracy of the converter will drop. This is because the current command is determined by the estimated rotor position, in which case the synchronous machine cannot produce a stable output.
It is therefore an object of the present invention to drive a synchronous generator through a simple sensorless control with a short calculation time and without using a sensor for detecting the position of a rotor, accurately control a converter independently of an error in the estimated value of the rotor position, and produce a stable output of the generator.
In accordance with the present invention, the above object is attained by providing an apparatus for controlling a synchronous generator which comprises a converter connected to a stator of the synchronous generator and an inverter connected to the converter and also connected to a power system. The converter converts a power of a variable frequency generated by the synchronous generator to a D.C. power, and the inverter converts the D.C. power to an A.C. power of a fixed frequency. The converter includes a voltage detector for detecting a terminal voltage of the stator of the synchronous generator, a current detector for detecting a current flowing through the stator of the synchronous generator, and a rotor position estimating means for estimating a rotor position of the synchronous generator from a voltage detected by the voltage detector and a current detected by the current detector. The rotor position estimating means calculates a d-axis component of a voltage induced in the synchronous generator in a d-q axis coordinate system determined by a rotor position during estimation and controls the d-axis component of the induced voltage to zero to estimate the rotor position. The rotor position estimating means includes an active power detector for detecting an active power of an output of the synchronous generator and a reactive power detector for detecting a reactive power of the output of the synchronous generator or a terminal voltage detector for detecting an effective value of the terminal voltage of the stator of the synchronous generator The rotor position estimating means also includes an active power controller for controlling the active power of the output of the synchronous generator with use of the active power value detected by the active power detector and an active power controller for controlling the reactive power of the output of the synchronous generator or a terminal voltage controller for controlling the terminal voltage of the synchronous generator with use of the terminal voltage value detected by the terminal voltage detector. The active power controller adjusts a q-axis current command on the d-q axis coordinate system determined by the rotor position estimating means to control the active power of the synchronous generator. The reactive power controller or the terminal voltage controller adjusts a d-axis current command on the d-q axis coordinate system determined by the rotor position estimating means to control the reactive power or terminal voltage of the synchronous generator.